Axel Heilberg's Vanished Forests

Fossil mystery: How did giant redwoods live near the
North Pole?

By Michael PurdyHomewood

Once upon a time, Axel Heilberg Island was a very
strange place. Located in the Arctic Circle north of Canada,
a full eight-ninths of the way from the equator to the North
Pole, the uninhabited Canadian island is far enough north to
make Iceland look like a great spot for a winter getaway,
and today there's not much to it beyond miles of rocks, ice,
a few mosses and many fossils.

The fossils tell of a different era, though, an odd
time about 45 million years ago when Axel Heilberg, still as
close to the North Pole as it is now, was covered in a
forest of redwoodlike trees known as metasequoias.

Hope Jahren, an assistant professor of
Earth and Planetary Sciences in the
Krieger School of Arts and Sciences at Johns Hopkins,
recently published results that partially demystified Axel
Heilberg's vanished forests. Jahren and colleague Leonel
Silveira Lobo Sternberg of the University of Miami uncovered
evidence that the Axel Heilberg's forests probably received
equatorial water and warmth from a prehistoric weather
pattern unlike anything in existence today.

Hope Jahren on Axel
Heilberg, where she holds a fossil metasequoia, one of the
hundreds her group has excavated there. Jahren, who recently
received the Geological Society of America Donath medal for
most promising young scientist, has made three trips to the
island.

Other challenging mysteries remain, including how a
forest could develop given the sunlight it would receive on
Axel Heilberg. Because of its closeness to the North Pole
both now and in the time of the redwoods, Axel Heilberg
spends four months of each year in continuous sunlight and
four months of each year in continuous darkness.

"We don't have plants that can survive under those
conditions today, let alone forests," Jahren says. "For a
tree to endure four months of daylight is like you or I
going without sleep for four months."

Through a grant from the Andrew Mellon Foundation,
Jahren's research group has made three summer visits to Axel
Heilberg, excavating hundreds of fossil metasequoias. The
fossils are immaculately well-preserved.

"Some of this stuff looks about like driftwood on the
beach, but it's 45 million years old," Jahren says. "These
fossils are chemically preserved at a level you usually
would expect to see in something that's only 1,000 years
old."

That's ideal for Jahren, who studies the presence of
isotopes of elements like carbon, nitrogen and oxygen in
living and fossilized plants. Isotopes are forms of an
element that differ only by the addition of one or more
subatomic particles known as neutrons. Different isotopes of
the same element have different mass, a property that
affects the way plants use them.

Jahren studies the isotopes to learn more about plants'
relationships to weather and climate change. She won last
year's Geological Society of America Donath Medal for most
promising young scientist, receiving the award at a ceremony
in Boston in front of colleagues and several of her former
teachers.

"It was really a beautiful moment," recalls Jahren,
glowing with happiness as she describes the ceremony. "To
find out after so many nights, weekends and years spent on
research that people really do care, and it really does
matter."

Other GSA medals were given at the same ceremony, and
Jahren says she was amazed by the "stunning career
contributions" being recognized at the same time as her
work. The Donath medal, the GSA's most prestigious for a
young scientist, comes with a $20,000 award.

"It felt a little funny to receive it as one person,"
Jahren says. "I've always felt so strongly that what we
accomplish is done as a team, so I felt like one person
accepting on behalf of many students and colleagues."

In her group's first major Axel Heilberg results,
published in the January issue of
GSA Today, they measured the presence of isotopes
of oxygen and hydrogen in the fossilized metasequoias.

"The wood of any tree growing anywhere records fairly
faithfully the oxygen and hydrogen chemistry of the water
the plant has access to through precipitation," Jahren
explains. "And there's a great deal of difference between
the chemistry of water that arrives at a certain location
after being transported [in evaporated form] great distances
over land versus the chemistry of water that arrives at a
place after being transported over water or not being
transported very far."

Jahren and co-author Sternberg chemically compared the
fossil isotope levels with those found in water in
contemporary precipitation patterns over great distances of
forested lands in the Amazon. They were able to show that
water traveling from near the equator almost due north
across the continents to the vicinity of Axel Heilberg would
have signature levels of oxygen and hydrogen isotopes that
matched those found in the fossils.

While it might seem mind-boggling to have the equator
watering the North Pole, Jahren notes that other major
climatological differences at the time included the lack of
a north polar ice cap.

"It's very hard to explain the isotope chemistry of the
precipitation using any other model of water transport,"
Jahren says. "So we think we've basically solved a piece of
the puzzle."

As for the other major piece of the puzzle--survival of
the trees through extended periods of light and
dark--Jahren's group is working to see if the isotope
chemistry of the fossils can help them learn how the
metasequoias' metabolism compared to those of contemporary
plants.

"Did they function similarly to how plants function
now?" Jahren asks. "Or did they have strategies that plants
either no longer have or no longer employ? Were they
fundamentally different? These fossils are really forcing us
to expand our ideas of how ecosystems can work."